The following documents are believed to represent the current state of the art:
U.S. Pat. Nos. 5,217,530; 5,711,796; 5,827,568; 6,156,828; 6,562,118; 6,758,892; 7,241,337; 7,297,204;
U.S. Patent Publication Nos. 2001/0047738; 2002/0108534; 2004/0101365; 2008/0060551; 2008/0168926; 2008/0224345; 2008/0271639; 2009/0061236; 2009/0105376; 2010/0022686;
Published PCT Application: WO 2005/100479.